The intestine is an unique organ providing dietary and re-absorbed biliary cholesterol to the body. Excess cholesterol can result in cholesterol gallstone disease. Cholelithiasis is prevalent in cultures consuming a Western diet with high cholesterol, and can be induced in mouse models by a high cholesterol and cholic acid diet. Therefore, understanding cholesterol absorption is of great importance to both prevention and treatment of cholesterol gallstones. It has been known that genetic factors apparently play a critical role in the development of cholesterol gallstones in inbred mice. It has been found that differences in gallstone susceptibility between C57L and AKR strains are determined by at least two Lith (gallstone) genes, as well as that the sister of P-glycoprotein (Spgp), a canalicular bile salt transporter is a candidate gene for the Lith1. A recent observation that there is a remarkable positive correlation in eleven strains of inbred mice between percent cholesterol absorption and prevalence of cholesterol gallstones at 8 weeks of feeding the lithogenic diet strongly suggests that the extent of cholesterol absorption from the intestine may be a genetically determined step for cholesterol gallstone formation. Furthermore, it has been observed that there are gender differences in susceptibility to cholesterol gallstones, favoring males to females by 2:1. The applicant proposes five specific aims to explore genetic and physiological mechanisms of cholesterol homeostasis as well as molecular mechanisms of cholesterol absorption, and pathophysiological mechanisms of the formation of lithogenic bile and cholesterol cholelithiasis. Aim 1: To investigate genetic variations in cholesterol absorption efficiency and their role in cholesterol gallstone formation among 12 strains of inbred mice. Aim 2: To define differences in molecular mechanisms for cholesterol absorption and chylomicron formation between mice with high and low cholesterol absorption. Aim 3: To study postprandial chylomicron metabolism and its role in the lithogenesis of bile. Aim 4: Using genetically gallstone-susceptible and the SR-B1 att (knockout) mice, to elucidate the role of SR-B 1 (HDL) receptor in biliary cholesterol secretion and cholesterol gallstone formation. Aim 5: To characterize hormonal basis for gender differences in the gallstone phenotypes. These studies should provide important contributions to our basic understanding of cholesterol homeostasis as well as pathogenesis of cholesterol gallstone formation.